A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the bones of the middle ear 103 (malleus, incus, and stapes), which in turn vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. The cochlea 104 includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The scala tympani forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses that are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. For example, in some patients, the cochlear shape fails to develop properly and various malformation conditions can occur such as those shown in FIG. 2: cochlear aplasia, cochlear hypoplasia, common cavity malformation, and incomplete partitioning.
A cochlear implant is an auditory prosthesis which uses an implanted stimulation electrode to bypass the acoustic transducing mechanism of the ear and instead stimulate auditory nerve tissue directly with small currents delivered by multiple electrode contacts distributed along the electrode. FIG. 1 also shows some components of a typical cochlear implant system which includes an external microphone that provides an audio signal input to an external signal processing stage 111 where various signal processing schemes can be implemented. The processed signal is then converted into a digital data format, such as a sequence of data frames, for transmission into the implant stimulator 108. Besides extracting the audio information, the implant stimulator 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through connected wires 109 to an implanted electrode carrier 110. Typically, this electrode carrier 110 includes multiple electrodes on its surface that provide selective stimulation of the cochlea 104.
Cochlear implant systems need to deliver electrical power from outside the body through the skin to satisfy the power requirements of the implanted portion of the system. As shown in FIG. 1, an external transmitter coil 107 (coupled to the external signal processor 111) is placed on the skin adjacent to a subcutaneous receiver coil connected to the implant stimulator 108. Often, a magnet in the external coil structure interacts with a corresponding magnet in the subcutaneous secondary coil structure. This arrangement inductively couples a radio frequency (rf) electrical signal to the implant stimulator 108. The implant stimulator 108 is able to extract from the rf signal both the audio information for the implanted portion of the system and a power component to power the implanted system.
When the cochlea is severely ossified, double branch intra-scala electrodes have been used as shown in FIG. 3, where an ossified cochlea 301 receives two intra-scala electrodes 301 and 302 for stimulating auditory nerve tissue in different sections of the cochlea 301. As shown, this approach requires two cochleostomies for the electrode insertions. For a fuller discussion of this subject, see, for example, U.S. Pat. No. 5,922,017, which is incorporated herein by reference.
In the case of malformed cochleas such as the ones illustrated in FIG. 2, there may be some neural tissue present within the cochlear volume. In such cases, both conventional single branch and double branch multi-contact electrodes have been inserted into the cochlear fluid in the existing cavity and electrical stimulation signals applied. Similarly, in incomplete partition cochleas, conventional multi-contact electrodes have been used, usually having a shorter length.